The present invention relates generally to diagnostic imaging and, more particularly, to a method and apparatus of recovering missing projection data in an energy discrimination (ED) diagnostic imaging system.
Exemplary diagnostics devices comprise x-ray systems, magnetic resonance (MR) systems, ultrasound systems, computed tomography (CT) systems, positron emission tomography (PET) systems, and other types of imaging systems. It is well-understood that in medical imaging, it is necessary to measure the same attenuation path at least two times with different incident x-ray spectra, in order to obtain material specific information. Exemplary approaches to create distinct incident spectra for an energy discrimination CT system comprise: (i) image the same object at different tube kilovoltage potentials (kVp's); (ii) employ an energy sensitive detector to divide the same incident energy spectrum into several sub spectra; (iii) use a layered detector assembly such that each detector layer senses a different x-ray energy spectrum. Such CT systems have the ability to compute the conventional Hounsfield Unit (HU) value as well as a certain level of information about the material composition. The information about object material composition is typically obtained through “basis material decomposition (BMD).” In exemplary BMD, the measured dual-spectral projection data are re-mapped to the integrated density projection data corresponding to two basis materials.
Typically, in CT imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry opening within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
An exemplary implementation of a CT system comprises an energy integrating (EI) CT system. In the EI CT system, the detector gets a charge that is proportional to the integral of the area of the pixel exposed to the x-ray beam, the time of exposure, and the integral of the energy weighted spectra that hit the pixel. The EI detector integrates these three things simultaneously and gives a single charge that is then converted into a CT image.
Another exemplary implementation of a CT system comprises an energy discriminating (ED) CT system. Exemplary photon counting and ED detectors have limited count rate. Below the limited count rate, the ED detector can record the energy of the incident photons, or the integral counts corresponding to a given energy region, often call an “energy bin”. There could be multiple energy bins provided by an ED detector, each of which covers a targeted energy region of the incident x-ray photons. To use these detectors for medical or other high-flux CT applications, the CT system has to deal with the loss of ED detector readouts, where the incoming x-ray photon count rate exceeds the limit of the ED detectors.
Another exemplary implementation of a CT system comprises main and secondary detectors, each with an associated x-ray tube. The main detector covers the field of view (FOV), while the secondary detector has a limited FOV. This system improves the temporal resolution while the tubes are operated at the same kVp. The missing data in the secondary detector are patched with the data from the main detector. To operate the tube at different kVp's for material decomposition, the missing data in the secondary detector cannot be patched directly with the data from the main detector, due to the kVp inconsistency. The effective FOV with the dual-kVp technique is restricted by the secondary detector, since the data recovery algorithm provides an inadequate patch.
Therefore, it would be desirable to promote a decrease in effects of limitations of photon counting in an ED diagnostic imaging system.